High frequency solid state switch employing diodes with shiftable bias to control signal transmission



Aug. 25, 1964 J. w. SPALLQNE 3,

HIGH FREQUENCY SOLID STATE SWITCH EMPLQYING DIODES WITH SHIFTABLE BIAS TO CONTROL SIGNAL TRANSMISSION Filed Feb. 23, 1962 2o SWITCH SECTION swrrcH SEQTION John W. Spollone INVENTOR United States Patent This invention relates to a solid state switch adapted for use at high frequencies. More specifically, it relates to a switch in which series-and-parallel-con'nected diodes,

in the'tran smissio'n path are biased in the forward or re- 5 verse direction depending on whether or not transmission along the path is desired. The switch is particularly well suited for use with transmission lines.

The so-called solid state" switch, a device using semiconductors to replace the physically moveable contacts of conventional switches, has several attributes which make it desirable for use in high frequency systems. For example, the switching time is less by several orders of magnitude. Also, there is no problem of contact wear or entry of foreign matter between contacts, factors which are particularly troublesome at high frequencies.

A principal object of the present invention is to provide an improved solid state switch for use at high frequencies.

Another object of the inventionis to provide. a switch of the above type having a short switching time and pro viding a high degree of reliability.

A further object of the invention is to provide a switch of the above type characterized by compatibility with conventional transmission lines.

Yet another object of the invention is to provide a switch of the above type having a low insertion loss.

A still further object is to provide a switch of the above type characterized by simplicity and relatively low cost of construction. y a

Another object is to provide a switch which may be operated in a reverse direction with equal efficiency.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a single throw switch embodying the present invention, and

FIG. 2 is a simplified schematic diagram of a double throw switch.

Basically, a single section switch embodying our invention comprises three diodes connected as a T network, two of the diodes being in series with the line and the other one in parallel or across it. Transmission through the switch section takes place when the series diodes are biased to present a low impedance and the parallel diode presents a high impedance. Conversely, a shift of the bias voltages to provide a high series impedance and a low parallel impedance prevents transmission through the section.

The useof diodes in a T configuration is not new with our switch. Rather, it is the manner in which'the switching diodes are interconnected with other circuit elements that is the subject of the invention. Our circuit arrangement, which is described in detail below, is simple and yet it provides a short switching time together with a low insertion loss when the switch is conducting and a high degree of isolation when it is open.

More specifically, as seen in FIG. 1, a switch 8 embodying our invention includes a pair of series diodes 10 and 12 and a parallel diode 14 connected together at a junction 16. Like electrodes of the diodes 10 and 12 are connected to the junction and the opposite electrode of the diode 14 is connected thereto. In the illustrated circuit, these electrodes are, respectively, the cathodes of the diodes 10 and 12 and the anode of the diode 14.

The anodes of the diodes 10 and 12 are connected to the inner conductors of transmission line connectors schematically indicated at 18 and 20. The cathode of the diode 14 is connected to the other side of the transmission line, i.e., the outer conductors of the connectors 18 and 20, by way of a ground connection through a capacitor 22. A resistor 24 in parallel with the capacitor 22 provides a direct current path through the diode 14 to ground. A resistor 26 having one end connected to the junction 16 is connected by way of a by-pass capacitor 28 to a switch,

generally indicated at 30. The switch 30, in turn, is connected to a power supply, illustratively depicted as a pair of batteries 32 and 34, in such manner that manipulation of this switch changes the polarity of the power supply voltage applied to the terminal 36 at which the switch is connected to the resistor 26.

With further reference to FIG. 1, assume that the switch 30 is in the position shown, so that a negative potential is applied to the terminal 36. The junction 16 is then also negative and therefore the diodes 10 and 12 conduct by way of inductors 38 and 40. Thus, their operating points are on the lower resistance, forward portions, of their resistance characteristics and they present a low impedance between the connectors 18 and 20. At the same time the negative potential at junction 16 reverse biases the diode 14, which is thus in the high resistance, reverse current portion of its characteristic. This effectively isolates the junction 16 from ground. The switch 8 therefore has a low series impedance and at the same time a low parallel admittance, the requisites for efiicient transmission between the connectors 18 and 20.

If the position of the switch 30 is reversed, a positive voltage is applied to the terminal 36 and therefore also to the junction 16. This reverse biases the diodes 10 and 12, thereby effectively cutting them off, and at the same time it causes forward conduction through the diode 14 to provide a low impedance path therethrough. The capacitor 22 bypasses the resistor 24 at the transmission line frequency, thus providing a low impedance path across the line from the junction 16 to ground.

The battery voltages and the resistances of the resistors 24 and 26 are preferably chosen so that when the switch 8 conducts, the currents through the series diodes 10 and 12 are sufiicient to provide operation along a portion of the diode characteristic having a relatively low incremental resistance. This, in combination with the high resistance of the reverse-biased diode 14, provides a low insertion loss. The internal capacitance of diode 14 does not adversely affect insertion loss. In fact, in some cases,

Patented Aug. 25, 1964 we prefer to include a capacitor 42 between the junction 16 and ground for additional capacitance so as to provide a desirable characteristic impedance in the switch.

The resistors 24 and 26 are so proportioned that a substantial reverse bias is imposed on the series diodes and 12 when the switch is open." The internal capacitance of a junction diode decreases as the reverse bias is increased and therefore, within limits, the series impedance in the switch is increased by an increase in bias voltage. There is an upper limit on the reverse bias which results from the fact that once a certain point is reached the incremental resistance decreases with increasing voltage;

By way of illustration, we have found the following circuit values to provide highly satisfactory operation of the switch 8:

Diode 10 2 type CGD 1062 diodes connected in parallel.

With the above circuit values we have obtained an insertion loss of less than 2 db at 200 megacycles when the switch 8 is in the conducting state and isolation of greater than 55 db when it is in the non-conducting state. A switching speed of 50 nanoseconds is readily obtained.

It will be observed that since the batteries 32 and 34 have the same voltage they may be replaced by a single battery connected between the terminal 36 and ground by a polarity-reversing switch 30. The bias switch 30 may be an electronic switch and such construction is, of course, highly desirable when high speed operation is contemplated.

In FIG. 2 we have illustrated a double throw switch embodying our invention. The switch includes sections 44 and 46, each of which has the same construction as the switch 8 of FIG. 1. The sections 44 and 46 share the same conector 18 and inductor 38 at one end and at their other ends they have separate connectors 20 and 20 respectively. Their bias terminals 36 and 36' are connected to the batteries 32 and 34 by a double pole, double throw switch generally indicated at 48.

In the position of the switch 48 shown in FIG. 2, positive potential of the battery 34 is applied to the junction 36 and the negative potential of the battery 32 is applied to the junction 36. Accordingly, the connector 20' of the section 46 is connected to the connector 18 and the connector 20 of the section 44 is isolated therefrom. Reversal of the switch 48 provides transmission between the connectors 18 and 20 and isolation of the connector 18 from the connector 20'.

It will be apparent that more switch positions may be provided by means of additional switch sections connected to the connector 18 of FIG. 2. Alternatively, each of the terminals 20 and 20 may have a further pair of switch sections connected thereto according to the well known Christmas tree switch configuration.

Each switch is preferably housed in a metallic box with the series elements extending through the box in the manner of a transmission line inner conductor. By way of example, each section of a multiple section switch may occupy a compartment having a length of 2.75 inches, :1 width of 1.25 inches and a height of 0.75 inch. Cross partitions in each compartment aid in supporting the various components. It is significant to note that the cross partitions also serve a highly important shielding function. The switch is preferably potted, a suitable potting material being Eccofoam, made by the Emerson and Cumings company, Canton, Massachusetts. With this potting material and a value of 2.7 picofarads for the capacitor 42, the above physical arrangement can provide a close match to a 50 ohm transmission line, the voltage standing wave ratio being about 1.1 with the inductor characteristics set forth below. Where higher capacitance potting materials are used, the capacitor 42 may be eliminated.

More specifically,'the frequency of the signal handled by the switch is preferably above the self-resonant frequency of the inductors 38 and 40. The inductors thus behave as capacitors at the signal frequency while providing the desired D.-C. paths to ground. The resultant capacitive loading within the switch aids in approximating characteristics of the transmission lines connected to the switch terminals.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. A high frequency switch comprising:

(a) first, second and third terminals,

(b) first and second diodes connected in series between said first and second terminals and connected together at a junction,

(c) said first and second diodes having the same direction of conduction with respect to said junction,

(d) a third diode,

(e) a first resistor in series with said third diode between said junction and said third terminal,

(f) the direction of conduction of said third diode with respect to said junction being opposite to that of said first and second diodes,

(g) a first capacitor in parallel with said first resistor and having an impedance substantially less than the impedance of said first resistor at the frequency of the signals transmitted by said switch,

(It) a second resistor connected between said junction and a fourth terminal,

(i) a voltage source,

(j) means for applying voltage from said source to said third and fourth terminals alternatively in opposite polarities,

(k) a second capacitor connected between said junction and said third terminal,

(I) a first inductor connected between said first terminal and said third terminal,

(m) a second inductor connected between said second terminal and said third terminal.

2. A multiple section switch comprising:

(a) a first terminal,

(b) a plurality of second terminals,

(0) switch sections connected between said first terminal and the respective second terminals,

(d) each of said switch sections comprising:

(1) first and second diodes connected in series be tween said first terminal and the second terminal connected to said section,

(2) each of said first and second diodes conducting in the same direction with respect to said junction,

(3) a third diode,

(4) a first resistor in series with said third diodes between said junction and a third terminal,

(5) the direction of conduction of said third diode with respect to said junction being opposite to that of said first and second diodes,

(6) a capacitor in parallel with said first resistor and having a substantially lower impedance than said first resistor at the frequency transmitted by said switch,

(7) a second resistor connected between said junction and a fourth terminal,

(e) a voltage source,

(1) switching means for applying voltage from said source to the third and fourth terminals of the respective sections with variable predetermined polarities.

3. The combination defined in claim 2 in which said third terminals are connected together and including an inductor connected between said first terminal and said third terminals.

4. The combination defined in claim 3 including an inductor connected between each of said second terminals and said third terminals.

References Cited in the file of this patent UNITED STATES PATENTS 2,535,303 1 Lewis Dec. 26, 1950 2,576,026 Meacham Nov. 20, 1951 2,782,307 Von Silvers et a1 Feb. 19, 1957 

1. A HIGH FREQUENCY SWITCH COMPRISING; (A) FIRST, SECOND AND THIRD TERMINALS, (B) FIRST AND SECOND DIODES CONNECTED IN SERIES BETWEEN SAID FIRST AND SECOND TERMINAL AND CONNECTED TO GETHER AT A JUNCTION, (C) SAID FIRST AND SECOND DIODES HAVING THE SAME DIRECTION OF CONDUCTION WITH RESPECT TO SAID JUNCTION, (D) A THIRD DIODE, (E) A FRIST RESISTOR IN SERIES WITH SAID THIRD DIODE BEBETWEEN SAID JUNCTION AND SAID THIRD TERMINAL, (F) THE DIRECTION OF CONDUCTION OF SAID THIRD DIODE WITH RESPECT TO SAID JUNCTION BEING OPPOSITE TO THAT OF SAID FIRST AND SECOND DIODES, (G) A FIRST CAPACITOR IN PARALLE WITH SAID FIRST RESISTOR AND HAVING AND IMPEDANCE SUBSTANTIALLY LESS THAN THE IMPEDANCE OF SAID FIRST RESISTOR AT THE FREQUNCY OF THE SIGNALS TRANSMITTED BY SAID SWITCH, (H) A SECOND RESISTOR CONNECTED BETWEEN SAID JUNCITION AND A FOURTH TERMINAL, (I) A VOLTAGE SOURCE, (J) MEANS FOR APPLYING VOLTAGE FROM SAID SOURCE TO SAID THIRD AND FOURTH TERMINALS ALTERNATIVELY IN OPPOSITE POLARITIES, (K) A SECOND CAPACITOR CONNECTED BETWEEN SAID JUNCTION AND SAID THIRD TERMINAL. (L) A FIRST INDUCTOR CONNECTED BETWEEN SAID FIRST TERMINAL AND SAID THIRD TERMINAL, (M) A SECOND INDUCTOR CONNECTED BETWEEN SAID SECOND TERMINAL AND SAID THIRD TERMINAL. 